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The invention relates generally to a method for quantifying sample cleanup in real time in a wellbore environment. Specifically, the invention is a method of curve-fitting measurements of physical properties measured for fluid samples being pumped from a formation surrounding a wellbore by a wireline or monitoring while drilling formation tester to predict the time at which a sample having a desired purity can be obtained.
In wellbore exploration, typically drilling muds such as oil-based muds and synthetic-based muds or water-based muds are used. The filtrates from these muds generally invade the formation through the borehole wall to an extent, meaning that this filtrate must be removed from the formation in order to access the formation fluids. Open hole sampling is an effective way to acquire representative reservoir fluids. Sample acquisition allows determination of critical information for assessing the economic value of reserves. In addition, optimal production strategies can be designed to handle these complex fluids. In openhole sampling, initially, the flow from the formation contains considerable filtrate, but as this filtrate is drained from the formation, the flow increasingly becomes richer in formation fluid. That is, the sampled flow from the formation contains a higher percentage of formation fluid as pumping continues.
It is well known that fluid being pumped from a wellbore undergoes a cleanup process in which the purity of the sample increases over time as filtrate is gradually removed from the formation and less filtrate appears in the sample. As the composition of the sampled formation fluid changes, so do the optical and physical properties of the sampled fluid, such as optical absorption, fluorescence, refractive index, viscosity, density, sound speed, and bulk modulus. A number of different measurements are used to determine various optical and physical properties of a fluid downhole in real time. Measuring these properties of the fluid therefore provides insight into a sample""s purity.
When extracting fluids from a formation, it is desirable to quantify the cleanup progress, that is, the degree contamination from filtrate in real time. If it is known that there is too much filtrate contamination in the sample (more than about 10%), then there is no reason to collect the formation fluid sample in a sample tank until the contamination level drops to an acceptable level. On the other hand, if by pumping for very long time, it is possible to achieve only slightly better contamination level, an operator ends up wasting very expensive rig time and also risks the very costly possibility of allowing a tool to become stuck in the wellbore. Thus, there is a need to determine how long one must pump to obtain a suitable purity sample from the formation.
When pumping first begins, the fluid being pumped contains a large amount of mud filtrate contamination and the fluid filtrate percentage is decreasing at the fastest rate. This process of decreasing fluid filtrate contamination is referred to as sample clean up. Later, the pumped fluid contains less contamination and the fluid filtrate percentage decreases at a slower rate. Oliver Mullins, Jon Schroer, and G. Beck have a published paper on curve fitting of sampled absorbance values versus time data to quantify clean up in real time, xe2x80x9cReal Time Determination of Filtrate Contamination During Openhole Wireline Sampling by Optical Spectroscopy,xe2x80x9d SPWLA, 41st Annual Meeting, Dallas, Tex. June 2000. See also, U.S. Pat. Nos. 6,274,865 and 6,350,986.
In this paper, Mullins et al. assume that the rate of sample cleanup as measured by observing optical density progresses as txe2x88x925/12 where t is time. This clean up rate is based on empirical experience in the Gulf of Mexico and elsewhere. However, Mullins et al. also state that, for extended pumping durations, they believe that the sample cleanup rate for shallow invasion progresses as txe2x88x921/3 and that the cleanup rate for deeper invasions progresses as txe2x88x922/3. Clearly, an assumption of a sample clean rate of txe2x88x925/12 can be rigid and inapplicable to real time situations. Moreover, using time as a fitting parameter necessarily assumes a constant pumping rate. Another problem with monitoring sample clean up over time by looking at optical absorption over time is that sand particles and other particulates can cause considerable scattering, which causes the absorption values measured over time to xe2x80x9cjumpxe2x80x9d and appear noisy. Thus, there is a need for a more flexible model so that curve-fitting can provide additional usable information regarding formation properties and characteristics for downhole pumping in real time.
The present invention provides a method of quantifying sample clean up in real time by curve-fitting at least two measurements over time or over volume of the same optical or physical properties of fluid samples taken from a formation surrounding a borehole. Sample fluid is extracted from the formation surrounding the borehole. As fluid continues to be extracted from the formation, the composition of the sampled fluid changes, altering the measured values of an optical or physical property for the sampled fluid. The present invention regresses measurements of the sampled formation fluid properties over time, to fit the preferred functional form to the acquired measured data points. In a preferred embodiment of the invention, iterative methods are provided that enable a user to determine an asymptotic value of a physical property, e.g., absorbance, as well as the percent that the current sample has achieved toward reaching the asymptotic value. The present invention also provides a projected time or volume to reach the asymptotic value. If the projected time or volume required to reach that asymptotic value is determined to be excessive, the operator may abandon extracting fluid from the formation region. In another embodiment of the present invention, a more general method enables the user to estimate, from the value of a fitted parameter, the speed at which cleanup will occur. In another embodiment, the measurements are indicative of formation physical properties such as formation damage.